Fine grained cemented carbide with refined structure

This is a continuation application of U.S. patent application Ser. No. 12/129,963, filed May 30, 2008, now U.S. Pat. No. 7,938,878, which claims benefit of SE 0701320-4 filed Jun. 1, 2007; and SE 0701449-1, filed Jun. 15, 2007, which are incorporated by reference herein in their entirety.

The present invention relates to a fine grained cemented carbide containing chromium. By adding small but well controlled amounts of Ti, Nb, V, Zr or Ta alone or in combinations, a further grain refined cemented carbide structure has been obtained without embrittling secondary phases.

Cemented carbide cutting tools with a grain refined structure are used today to a great extent for the machining of steel, hardened steel, stainless steels and heat resistant alloys in applications with high demands on both toughness and wear resistance. Another important application is in microdrills for the machining of printed circuit board, so-called PCB-drills. In these types of applications it is known that the amount and size of defects, such as abnormal WC-grains, small precipitates of embrittling phases, porosity, binder phase lakes, are critical for the mechanical properties, e.g. fracture toughness and transverse rupture strength (TRS).

Common grain growth inhibitors include vanadium, chromium, niobium, tantalum or compounds involving these. When added, generally as carbides and in an amount of from about 0.15 to about 1.5 wt-%, they limit grain growth during sintering, but they also have undesirable side effects, affecting the toughness behaviour in an unfavourable direction. Vanadium is known to be the most effective grain growth inhibitor but is also particularly detrimental due to its negative influence on the toughness properties. Normal levels of vanadium additions are also known to result in precipitation of embrittling phases in the WC/Co grain boundaries.

In order to achieve a fine, as sintered grain size of less than about 0.6 μm for cemented carbides with Cr/Co-ratios of from about 0.04 to about 0.06, very fine WC raw material is needed and the sintering is generally performed at a relatively low temperature, e.g. close to a temperature of about 1360° C. and often by sinterHIP or followed by hot isostatic pressing in order to obtain a dense structure. Such a production route of course increases the production cost, but also increases the risk of uneven binder phase distribution. In addition, the use of these very fine WC-raw materials gives powders with inferior compaction properties, resulting in powders that are difficult to compact to complicated geometries such as cutting inserts.

According to WO 99/13120, the amount of grain growth inhibitors can be reduced if a carbon content of the cemented carbide close to eta-phase formation is chosen.

JP-A-11-152535 discloses a process to manufacture fine grained tungsten carbonitride-cobalt hard alloys using tungsten carbonitride as a raw material. JP-A-10-324942 and JP-A-10-324943 disclose methods to produce fine grained cemented carbide by adding the grain growth inhibitors as nitrides. In order to avoid pore formation by denitrification of the nitrides sintering is performed in a nitrogen atmosphere.

EP-A-1500713 discloses a method of making a fine grained tungsten carbide-cobalt cemented carbide comprising mixing, milling according to standard practice followed by sintering. By introducing nitrogen at a pressure of more than 0.5 atm into the sintering atmosphere after dewaxing but before pore closure a grain refinement including reduced grain size and less abnormal grains can be obtained.

EP-A-1803830 discloses cemented carbides which include WC having an average particle diameter of <0.3 μm as a hard phase and 5.5 to 15 wt-% of at least one iron group metal element as a binder phase, and comprise, in addition to the above hard phase and binder phase, 0.005 to 0.06 wt-% of Ti, Cr in a weight ratio relative to the binder phase of 0.04 to 0.2. Especially, the above cemented carbides contain no Ta.

According to US 2006/029511, there is provided a method of making a fine grained tungsten carbide-cobalt cemented carbide comprising mixing, milling according to standard practice followed by sintering. By introducing nitrogen at a pressure of more than 0.5 atm into the sintering atmosphere after dewaxing but before pore closure a grain refinement including reduced grain size and less abnormal grains can be obtained.

It is an object of the present invention to avoid or alleviate the problems of the prior art.

It is further an object of the present invention to provide fine or submicron cemented carbide cutting tools with improved binder phase distribution and reduced amounts of abnormal WC grains giving cemented carbide tools with improved mechanical and cutting properties.

In one aspect of the invention, there is provided a WC-Co cemented carbide comprising a composition of from about 3 to about 15 wt-% Co and Cr such that the Cr/Co-ratio by weight is from about 0.05 to about 0.15 and balance WC and in addition ppm levels of

one of the following additional elements V, Nb, Zr, Ta then with a grain size of WC of from about 0.1 to about 2.0 μm or

mixtures of at least two of Ti, V, Nb, Zr, Ta, then with a grain size of WC of more than about 0.3 up to about 2.0 μm,

the ratio of Me/Co=(at-% Ti+at-% V+at-% Nb+at-% Zr+at-% Ta)/at-% Co is lower than or equal to about 0.014−(CW_Cr)*0.008 and higher than about 0.0005, with a CW_Cr ratio of from about 0.79 to about 0.95, with CW_Cr=(magnetic-% Co+1.13*wt % Cr)/wt % Co where magnetic-% Co is the weight percentage of magnetic Co and wt-% Co is the weight percentage of Co in the cemented carbide.

In another aspect of the invention, there is provided a method of making a WC-Co cemented carbide as described in the preceding paragraph comprising mixing and wetmilling of powders of WC and Co, pressing and sintering wherein ppm levels of Ti, V, Nb, Zr or Ta or mixtures thereof, except Ti alone, as pure metals or as a carbides, nitrides and/or carbonitrides or mixtures thereof are added to the powder mixture.

FIG. 1 shows a back scattered SEM micrograph of the microstructure of a cemented carbide according to prior art.

FIG. 2 shows a back scattered SEM micrograph of the microstructure of a cemented carbide according to the invention.

It has now surprisingly been found that a pronounced grain refining effect in combination with an improved binder phase distribution can be obtained by the introduction of one of V, Nb, Zr, Ta or mixtures of at least two of Ti, V, Nb, Zr, Ta, on ppm-level in a chromium containing cemented carbide. The present invention provides a material that can be sintered at about 1360° C. or higher maintaining the fine grain size and binder phase distribution despite that the higher Cr/Co-ratio decreases the melting temperature of the binder and, thus, gives a longer time in the liquid state during the sintering where grain growth is rapid. The present invention allows for the same grain size to be achieved also allowing for higher carbon balances applicable for CVD coating.

The cemented carbide body comprises tungsten carbide with from about 3 to about 15 wt-% Co and Cr such that the Cr/Co-ratio by weight is from about 0.05 to about 0.15 and balance WC. In addition to that ppm levels of:

one of the following additional elements V, Nb, Zr, Ta then with a grain size of WC of from about 0.2 to about 2.0 μm, or

mixtures of at least two of Ti, V, Nb, Zr, Ta, then with a grain size of WC of more than about 0.3 up to about 2.0 μm. The ratio of

Me/Co=(at-% Ti+at-% V+at-% Nb+at-% Zr+at-% Ta)/at-% Co

is lower than or equal to about 0.014−(CW_Cr)*0.008 and higher than about 0.0005, preferably higher than about 0.0007, with CW_Cr from about 0.79 to about 0.95, preferably from about 0.80 to about 0.92, where

CW_Cr=(magnetic-% Co+1.13*wt % Cr)/wt-% Co

where magnetic-% Co is the weight percentage of magnetic Co and wt-% Co is the weight percentage of Co in the cemented carbide. The CW_Cr is a function of the W content in the Co binder phase. A CW_Cr of about 1 corresponds to a very low W-content in the binder phase and a CW_Cr of 0.75-0.8 corresponds to a high W-content in the binder phase.

The sintered body may also contain small amounts of precipitations of further additional phase or phases such as eta-phase, MX or M 7 X 3 , M 3 X 2 where M=(V+Zr+Ti+Ta+Nb+Co+Cr+W) and X=C or N may be allowed to a volume fraction of maximum 0.5 vol % without detrimental effects.

In one embodiment, the further additional element is Zr alone or in combination with one or more of the aforesaid additional elements.

In another embodiment, the further additional elements are Zr and Ti.

In yet another embodiment, the further additional elements are Ta and Ti.

In another embodiment, the cemented carbide contains from about 0.01 to about 0.10 wt-% N.

In yet another embodiment, the further additional element(s) is a mixture of Ti and Ta, having a N-content of more than about 0.02 wt-%, preferably more than about 0.03 wt-% but less than about 0.1 wt-%.

The method of making the cemented carbide according to the present invention with a composition according to above comprises mixing, milling, pressing and sintering of submicron tungsten carbide-cobalt bodies according to standard practice. Ppm levels of Ti, V, Nb, Zr or Ta or mixtures thereof, except Ti alone, are added as pure metals or as carbides, nitrides and/or carbonitrides or mixtures thereof in such amounts that the Me/Co and CW values according to above are fulfilled in the sintered cemented carbide.

The invention is additionally illustrated in connection with the following examples, which are to be considered as illustrative of the present invention. It should be understood, however, that the invention is not limited to the specific details of the examples.